The Earth’s oceans hold a vast array of environments, from sunlit tropical reefs to the crushing darkness of the abyssal plains. The polar seas represent the planet’s ultimate cold-water challenge. The boundary between liquid water and solid ice is a delicate balance of temperature, pressure, and salt content. Exploring the world’s coldest marine regions offers a look into the unique physical processes that push the limits of liquid water.
Identifying the World’s Coldest Sea
The title of the coldest sea on Earth belongs to the Weddell Sea, a large bay in the Southern Ocean surrounding Antarctica. It is positioned primarily between the Antarctic Peninsula and Queen Maud Land, forming an immense, deep embayment. It is bounded by the Filchner-Ronne Ice Shelf to the south and the coasts of Coats Land and the Antarctic Peninsula to the west.
The water masses in this region consistently hover near the freezing point of saltwater. Typical temperatures for the deep water, known as Antarctic Bottom Water (AABW), range from approximately -0.8°C to 0°C. Supercooled water beneath Antarctic glaciers can reach temperatures as low as -2.6°C. The Weddell Sea is a primary engine for forming the coldest and densest water mass on the globe.
The Mechanisms Driving Extreme Cold
The Weddell Sea remains liquid at temperatures below the freezing point of fresh water due to salinity. While pure water freezes at 0°C, dissolved salt depresses the freezing point to an average of about -1.8°C. The most intense cold is generated by brine rejection, which occurs during the rapid formation of sea ice in coastal polynyas.
When seawater freezes, water molecules solidify into ice crystals, but salt ions are largely excluded from the lattice. This rejected salt is flushed out, increasing the salinity and density of the surrounding liquid water. This newly densified, cold water is known as High-Salinity Shelf Water (HSSW) or Dense Shelf Water (DSW).
Massive ice structures, such as the Filchner-Ronne Ice Shelf, also contribute to the cold. The underside of these ice shelves interacts with the HSSW, further cooling the water. This extremely cold, dense water then sinks, cascading down the continental slope under gravity.
This sinking process forms the Antarctic Bottom Water (AABW), the coldest, densest water mass found in the global ocean. AABW formation in the Weddell Sea is a major component of the thermohaline circulation, a massive global current system sometimes called the ocean’s conveyor belt. As AABW flows northward along the seafloor, it ventilates the deep ocean basins, carrying oxygen and locking away heat and carbon for centuries.
Life in Sub-Zero Waters
Life in the Weddell Sea has evolved remarkable physiological mechanisms to survive water that can be below the internal freezing point of most organisms. Antarctic notothenioid fish, including the Jonah’s icefish, rely on specialized molecules called antifreeze glycoproteins (AFGPs). These proteins circulate in the fish’s blood, binding to small ice crystals and preventing them from growing larger and causing lethal freezing.
Some fish species, like the icefish, exhibit a unique adaptation, lacking red blood cells and hemoglobin. While this reduces their oxygen-carrying capacity, the extremely cold water holds a higher concentration of dissolved oxygen, sufficient for their slow metabolic needs. This allows oxygen to be dissolved directly into the plasma.
The region also demonstrates polar gigantism, where certain invertebrates grow to immense sizes compared to their warmer-water relatives. Examples include Giant Isopods and large sea worms found on the seafloor. Cold temperatures slow the metabolic rate of these cold-blooded animals, allowing them to live longer and grow larger.
Mammals like the Weddell seal also thrive in this environment, representing the southernmost breeding mammal on Earth. They are insulated by thick blubber and have developed complex diving adaptations to hunt in water near -1.9°C. Managing oxygen and blood flow during deep dives is essential for foraging for fish and krill beneath the extensive sea ice.